1. Field of the Invention
The present invention relates to toughened epoxy resin formulations useful for the manufacture of thermoset polymers for use in electronic packaging materials, other electronic applications, composites, or industrial applications.
2. Description of Background and Related Art
Epoxy resins are used to manufacture thermoset polymers which in turn can be use in various applications including composites, coatings, adhesives and electronic materials. For example, epoxy resins are commonly used in the electronics industry for making semiconductor packaging materials. Current epoxy resin formulations used in semiconductor packaging materials, include for example, high purity diglycidyl ether of bisphenol F or diglycidyl ether of bisphenol A along with high performance or multifunctional resins such as the digylcidyl ether of naphthalene diol or the triepoxide of para-aminophenol. The known epoxy resins suffer from balancing key attributes required for acceptable processability and downstream reliability. These attributes include viscosity, total and extractable chloride content, filler loading (for coefficient of thermal expansion (CTE) and modulus modification), adhesion, flux compatibility, toughness, dispense-ability, flow, and package level reliability performance including preconditioning, temperature cycle or shock, highly accelerated stress testing (HAST).
Conventional formulation approaches incorporate high purity bisphenol F or bisphenol A epoxy resins along with high performance or multifunctional epoxy resins. The inclusion of the high performance resins tend to increase the viscosity of the resultant blend negatively impacting the process-ability of the formulation, limiting both the amount and size of the particulate filler that can be incorporated into the formulation. Trends in electronic packaging designs toward smaller, stacked and high pitch configurations increase the demands on electronic packaging materials requiring better thermomechanical and processing performance. For example, underfill materials for electronic packaging need to have an even lower coefficient of thermal expansion (CTE) for resistance to thermal fatigue, while thermal interface materials (TIMs) need to be more thermally-conductive for cooling a heat-generating source while maintaining low viscosity with increased filler loadings.
Many electronic packaging materials are highly filled materials. The properties of the filled materials largely depend on the type of filler used and the level of filler loading. In general, increasing the filler loading level usually decreases the CTE while the modulus and thermal conductivity increase. Unfortunately, the viscosity of the highly filled material also tends to increase with an increase in filler loading.
During the application of these filled materials for electronic packaging, underfill encapsulants are required to have a low viscosity (e.g. less than 0.7 Pa·s at the dispense temperature) for adequate processing and complete, void-free, underfilling of the die. Therefore, it would be highly desirable to identify base formulation ingredients that enable low viscosity and acceptable thermomechanical properties (CTE, Tg, modulus, and in some cases, thermal conductivity). Ultimately, what is needed in the electronics industry is to develop formulated materials with low CTE (e.g. less than 30 ppm/° C.), high thermal conductivity (e.g. greater than 0.7 W/mK), moderate modulus (e.g. between 3 and 9 GPa) and proper flow in the case of underfill encapsulants (15-100 sec. across 15 mm in a 20 μm gap), the ability to tune the Tg after cure (e.g. 25-300° C.) for a specific application, all while maintaining acceptable rheology performance (application dependent).
Typical capillary underfill formulations incorporate the digycidyl ethers of bisphenol A or bisphenol F along with modifiers to improve the thermomechanical properties, such as glass transition temperature (Tg), of the cured system. For example, U.S. Pat. No. 7,842,201 teaches the use of Epon® 862 and Epon® 828 (Hexion Specialty Chemicals), along with Araldite® MY-0510 (Huntsman Advanced Materials) for nano-filled underfill encapsulants. The Epon® 862 is a bisphenol F based epoxy resin. The Epon® 828 is a bisphenol A based epoxy resin, and the Araldite® MY-0510 is 4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline. U.S. Pat. No. 7,279,223 discloses a number of aliphatic, cycloaliphatic, and aromatic epoxy resins. U.S. Pat. No. 7,351,784 teaches the use of cycloaliphatic amines and carbenes for capillary and no flow underfill formulations. Specific aliphatic amine structures cited are 4-(2-aminopropan-2-yl)-1-methylcyclohexanamine and 4,4′-methylenebis(2-methylcyclohexanamine).
The before mentioned epoxy resins are manufactured utilizing epichlorohydrin which is reacted with the phenolic hydroxyl group, or in the case of an aromatic amine, with the amino group resulting in the epoxy resin. During the coupling process, incomplete ring closure can occur resulting in bound or hydrolyzable chloride.
The hydrolyzable chloride content of the resin can have negative impact on the performance of the device or component during reliability testing, especially high humidity and high temperature testing such as pressure cooker exposure (PCT) 121° C./15 psi steam. During exposure to high humidity testing, the hydrolyzable chloride can be extracted from the cured polymer and form highly acidic species causing corrosion within the device. Therefore, it would be highly desirable to develop base resins that do not use epichlorohydrin or other halogenated reactants to manufacture the epoxy resin.
It is desired to provide an electronic packaging material that is a highly filled material with low viscosity and improved thermomechanical properties coupled with improved process ability and downstream package reliability and total chloride less than about 200 ppm, more preferably less than about 100 ppm, and most preferably less than about 5 ppm.